Dihydrofolate reductase is the site of action of several important drugs (i.e., methotrexate, trimethoprim) widely used in the chemotherapeutic treatment of numerous malignancies, certain bacterial infections, psoriasis, arthritis and other human diseases. Understanding the detailed mechanism of action of this enzyme and its inhibition by these drugs as well as its unique molecular biology should facilitate a rational approach to drug design and chemotherapy. Thus we continue our studies on the isolation and characterization of hepatic dihydrofolate reductase from various animals with the result that it was possible to compare the interaction of trimethoprim and methotrexate with the reductases from animals (chicken liver) and bacteria (E. coli). The correlation of binding and enzymatic inhibition with the 3-dimensional structure of the enzymes resulted in an explanation of the unique antibiotic properties of trimethoprim as compared with methotrexate. The development of a rapid HPLC procedure for carotinoids showed that eight major compounds are present in plasma; leutin, zeaxanthin, and unknown dihydroxy carotene, two isomers of cryptoxanthin, lycopene, Alpha-carotene and Beta-carotene.